Sorry for the late reply. Can you try to find a file called "dfu_transport_ble.c" and a define name "DEVICE_NAME" in your bootloader project? The dfu bootloader example will advertise as "DfuTarg" when building out of the box, I suppose this is the advertising name you want to change.

Sorry for the late reply. nRF5 SDK 12.3 is the latest SDK with support for the nRF51 series. The recommended setup for nRF51 series would then be SDK 12.3 + S130.

Unfortunately we didn't provide any migration guide before SDK 12.3. You could try to read the release notes for each SDK (you can select the SDK version in the dropdown menu up to the right) here, but as I suppose you already have experienced we only have documentation from SDK 9 and up... So maybe the easiest way would be to start from a clean template project from SDK 12.3 and then implement the modules used in your old project.

If you are planning to develop a new product and have plans to use the new features from Bluetooth 5 in the future I would also recommend taking a look at the nRF52 series.